In accordance with drastic development of various information devices ranging from a large-capacity server to a mobile terminal, there has been pursuits for even higher performances, for example, higher integration, higher speed, and lower power consumption, of their constitutional elements such as memories or logic elements. In particular, development of semiconductor non-volatile memories has been remarkable, and flash memories as large-capacity file memories have become widely available with such a momentum that hard disk drives are almost being eliminated. On the other hand, an FeRAM (Ferroelectric Random Access Memory), an MRAM (Magnetic Random Access Memory), and a PCRAM (Phase-Change Random Access Memory), and the like are being developed with a view to applying them to code storage use and further to working memories, in order to replace an NOR flash memory, a DRAM, or the like that are currently in general use. Some of these have been already put to practical use.
Among these, an MRAM is capable of high-speed and approximately infinite (1015 times or more) rewriting, because data storage is carried out by way of direction of magnetization of a magnetic body, and has been already used in fields of industrial automation or airplanes. It is expected that an MRAM will be developed in future for code storage or working memories because of high speed operation and reliability; however, in reality, there is a difficulty in lowering power consumption or increasing capacity. This is a difficulty originated from a recording principle of an MRAM, that is, a method in which magnetization is reversed by a current magnetic field generated from wirings.
As one method to solve the difficulty, there has been considered recording without the use of a current magnetic field, that is, a magnetization reversal system. In particular, researches in spin torque magnetization reversal are active (for example, refer to Patent Literatures 1, 2, 3, and Non-Patent Literatures 1, 2).
In many cases, a storage cell of spin torque magnetization reversal is configured of an MTJ (Magnetic Tunnel Junction) (TMR (Tunneling Magnetoresistive)) element, similarly to an MRAM.
This configuration utilizes that a spin polarized electron that passes through a magnetic layer pinned in a certain direction, when entering another free (with a direction unpinned) magnetic layer, gives torque to the magnetic layer (this is also called spin injection torque). Feeding a current of a certain threshold value or more allows the free magnetic layer to be reversed. Rewriting of 0/1 is carried out by changing polarity of a current.
An absolute value of a current for the reversal is 1 mA or less in an element having a scale of about 0.1 μm. In addition, the current value decreases in proportion to an element volume, which enables scaling. Furthermore, unlike an MRAM, a word line for generating a current magnetic field for recording may be eliminated, leading to an advantage of a simplified cell configuration.
In the following, an MRAM utilizing spin torque magnetization reversal will be referred to as an ST-MRAM (Spin Torque-Magnetic Random Access Memory). Spin torque magnetization reversal is also called spin injection magnetization reversal. There is a great expectation about an ST-MRAM as a non-volatile memory that enables lower power consumption and larger capacity while maintaining advantages of an MRAM, that is, high-speed and approximately infinite times of rewriting.